Collision Layer

Create a new Layer at the top and name it Collisions

Save the Image below. 

import it into Construct as a TileMap

Replace the default TileMap image.

In the TileMap Properties

Set the Tile Size to 64 x 64

Disable Initially Visible

Before you paint the level.

Double-click on each individual tile and  then:

Right-click on each individual tile in the map.

Guess the polygon shape.

Use the TileMap and block out the collision layer as shown

Once you have painted the TileMap 

move it between the Foreground and Background Layers

This allows us to see both the Level Design and the Collisions

Lastly, add the Solid behaviour to the TileMap